1. Field of the Invention
The present invention relates to a thermal transfer film, a thermal transfer recording medium, and a method for image formation using a combination of the thermal transfer film with the thermal transfer recording medium.
2. Prior Art
Thermal transfer can easily record variable information and thus is extensively used in a wide variety of applications. The thermal transfer is a method which comprises the steps of: putting a thermal transfer film, comprising a colorant layer provided on a substrate, on top of an object optionally provided with a receptive layer; pressing the assembly between a heating device, such as a thermal head, and a platen roll; and selectively heating the heating device in its heating portion according to image information to transfer the colorant contained in the colorant layer on the thermal transfer film onto the object, whereby an image is recorded on the object. Thermal transfer methods are roughly classified into thermal ink transfer (hot melt-type thermal transfer) and thermal dye sublimation transfer (sublimation-type thermal transfer)
The thermal ink transfer is a method for image formation wherein a thermal transfer film bearing thereon a heat-fusion ink layer is heated by the above heating means and the component of the softened heat-fusion ink layer is transferred onto an object such as natural fiber paper or plastic sheet to form an image. The heat-fusion ink layer used herein is formed of a dispersion of a colorant, such as a pigment, in a binder, such as heat-fusion wax or resin and is supported on a substrate such as a plastic film. The formed image has high density and high sharpness, and this method is suitable for recording binary images of characters, line drawings and the like.
On the other hand, the thermal dye sublimation transfer is a method for image formation wherein a thermal transfer film bearing thereon a sublimable dye layer is heated by the above heating means to sublimate and transfer the sublimable dye contained in the dye layer onto a receptive layer provided on an object, whereby an image is formed on the object. The sublimable dye layer used herein is formed of a solution or dispersion of a sublimable dye used as the colorant in a binder resin and is supported on a substrate film such as a plastic film. According to this method, since the amount of the dye transferred can be regulated dot by dot according to the quantity of energy applied to a heating device, such as a thermal head, the reproduction of gradation can be realized by varying the density.
Thus, the thermal ink transfer method and the thermal dye sublimation transfer method have respective features, that is, the thermal ink transfer method can easily and clearly form images of characters, numerals and the like, while the thermal dye sublimation transfer method is excellent in gradation rendering and can form images such as a photograph-like image of a face in a faithful, clear manner.
Images formed by the thermal ink transfer method, however, disadvantageously suffer from poor fastness or resistance properties, particularly poor abrasion resistance. On the other hand, images formed by the thermal dye sublimation method, unlike images using conventional printing ink, are free from any vehicle and thus are disadvantageously poor in fastness or resistance properties such as lightfastness, weathering resistance, abrasion resistance, and chemical resistance.
To overcome the above problems, for example, Japanese Patent Laid-Open No. 11674/1997 proposes the transfer of a protective layer onto information of a thermally transferred image to impart fastness or resistance properties to the image.
In the transfer of the protective layer, the protective layer should be evenly and accurately transferred in a specified size onto a specified position. To this end, in order to improve the sensitivity in the transfer of the protective layer, the glass transition temperature and the heat softening temperature of the resin constituting the protective layer are set to respective relatively low temperatures. As a result, the storage stability of the protective layer transferred sheet is deteriorated, and this poses problems including that blocking is likely to occur during storage of the protective layer transferred sheet in a roll form.